This invention relates to a position controlling apparatus adapted to control the position of an actuator using a shape memorizing alloy.
To control an actuator using a shape memorizing alloy body, there is already known a method of controlling about actuator with an information about the position of the actuator, the force applied to the actuator or the temperature by providing the actuator with a position detecting apparatus such as a potentiometer, a force detecting apparatus or temperature detecting apparatus.
In the conventional method of detecting the position or force of an actuator, the state of the actuator can be detected but the state of a shape memorizing alloy body which is a driving source, that is, the temperature of the shape memorizing alloy body can not be detected and therefore no judged whether the shape memorizing alloy is in a normal operating range or not.
In a method of detecting the temperature of a shape memorizing alloy body, a temperature sensor such as a thermistor is used but it is so difficult to make the sensor closely adhere to the shape memorizing alloy that the temperature of the shape memorizing alloy body can not be always correctly detected.
Therefore, with the conventional technique, the shape memorizing alloy is likely to be overheated and will deteriorate during recovering of the memorized shape and, when the alloy is further overheated, the memorized shape will be lost and the other parts forming the actuator will be damaged. When the shape memorizing alloy is overcooled, its fatigue will become so large that the characteristics will likely be deteriorated (such as in reduction in the lifespan.).
A position controlling apparatus effective to realize an actuator which is so small that a position detecting apparatus or the like can not be added to the outside in particular is shown, for example, in the publication of Japanese patent application laid open No. 33612/1985 and is characterized by detecting information relating to the displacement of the actuator by respective information about the resistance value of a shape memorizing alloy forming the actuator and the force applied to the actuator.
Now, in the article "Development of a Shape Memorizing Alloy Actuator" in Japan Robot Institute Journal, Vol. 4 No. 2 (April, 1986), in order to make it easy to handle a resistance value variation as a measure of a cotransformation rate, a normalized resistance value .lambda. is introduced. The normalized resistance value .lambda. is defined by the following formula where the maximum value of the resistance value R of a shape memorizing alloy body is represented by R max and the minimum value thereof is represented by R min ##EQU1## wherein R max&gt;R min.
That is to say, the normalized resistance value .lambda. is an index showing the rate of the mother phase occupied in the total phase so that, when shape memorizing alloys are competitively arranged and the respective normalized resistance values .lambda. are represented by .lambda..sub.1 and .lambda..sub.2, while satisfying the relation of .lambda..sub.1 +.lambda..sub.2 =1, the resistance values may be varied may be cooperatively controlled and may be favorably controlled by the reduction of the hysteresis.
However, in this conventional example, as the resistance value of the shape memorizing body is determined and the displacement of the actuator is determined by the resistance value, the following problems will be produced. That is to say, the resistance value of the shape memorizing alloy body will be low in reproductivity and will delicately vary due to the environment and fatigue. Therefore, in case the resistance value and displacement are made to correspond to each other, an error will be produced and no favorable control will be obtained.